1. There are a lot of security mitigations in modern Windows that add sometimes substantial runtime overhead, but offer significant protection. Writing an exploit for modern Windows is hard compared to XP SP1 and below.
2. The graphics are a lot more sophisticated. Animation and transparency and lots of good stuff.
3. There are a ton more "features" in the OS. Modern Windows is continually running a bunch of things that XP didn't.
4. All software tends to bloat over time. It's really hard and expensive to optimize things, especially in a huge and long-lived OS where the scope is enormous and PMs are constantly wanting things added. When it runs sufficiently on modern hardware, there's also very little incentive to do so. (As an aside, this is one of the reasons I love Linux. There's lots of incentive to optimize there and thus it does happen).
Overall I really miss XP. Surely nostalgia is a powerful drug, but damn that was a great OS.